Suspension means for string galvanometers



May 12, 1959 H. 1. CHAMBERS ETAL 2,836,782

SUSPENSION MEANS FOR STRING GALVANOMETERS Filed May 17, 1956 2 Sheets-Sheet 1 FIG].

RECORDING AMPLIFIER INVENTO HERBERT CHAMB E I PS ALBERT W FISCHER BY m MQM A TTORNEYS May 12, 1959 H. l. CHAMBERS ET AL 2,386,782

SUSPENSION MEANS FOR STRING GALVANOMETERS Filed May 17, 1956 2 Shets-Sheet 2 FIG. 3. l

i I ma /0/ v INVENTOR.

HERBERT l CHAMBERS By ALBERT W. FISCHER mmlzm A TTORNEYS United States Patent SUSPENSION MEANS FOR STRING GALVANOMETERS Herbert I. Chambers and Albert W. Fischer, Pasadena,

Calif, assignors to Consolidated Electrodynamics Corporation, Pasadena, Calif, a corporation of California Application May 17, 1956, Serial No. 585,568

Claims. (Cl. 324-154) The invention relates to string galvanometers and more particularly to suspension means for string galvanometers for recording an oscillographic trace directly upon a re cording medium such as current-sensitive paper.

String galvanometers of the direct writing type have peculiar problems stemming from the greater amplitude of string vibration necessary to record a discernible trace directly. The string or flexible conductor may be re quired to move as much as an inch from peak to peak and at frequencies in the range of 250 cycles per second. Conventionally, the flexible conductor is anchored at its opposite ends and connected electrically at these ends while the writing is done by the central portion of the conductor. The end mounting must be such that high frequency vibration of the conductor at the required large amplitudes may take place without fatigue to the conductor adjacent to the mounting. Additionally, the electrical connection of the ends of the conductor is preferably such that the connections are not placed under varying stresses due to the vibration of the conductor. If the electrical connections are subject to changing stress, the linearity of the response of the flexible conductor to the variation of the current input is adversely affected. Ideally, the mounting means provides a writing pressure against the recording medium that is substantially constant despite the relatively large amount of movement across the medium by the conductor.

We have developed suspension means for the flexible conductor in a string galvanometer which meets the requirements stated above. The invention is preferably embodied in a string galvanometer having a flexible conductor stretched through a magnetic field from anchors located at opposite ends of the conductor. Variations in voltages impressed on the conductor are manifested by changes in amplitude of vibration of the conductor in the magnetic field. A base which may be supported by the means for inducing the magnetic field in turn supports a first leaf spring near one of the base and a second leaf spring at its opposite end. Each leaf spring is fastened to the base by one end of the spring and each spring extends on an opposite side of the flexible conductor. Preferably the lines of extension of the first and second leaf springs from the sides of the conductor are substantially coplanar. The other end of each leaf spring is free. The flexible conductor is fastened at its respective ends to the base adjacent the respective fastened ends of the leaf springs and is stretched over the free ends of the leaf spring and extends between them. The leaf springs are pre-shaped to curve so that when a flexible conductor of a predetermined length is stretched over the springs, the resilient force of the springs imparts the desired degree of tension to the conductor. The conductor is preferably a cylindrical wire flattened at each of its ends for that portion which extends over the leaf springs. The greater width of the flattened portion extends in a direction that imparts more rigidity to the conductor perpendicular to the plane of deflection of the conductor.

The leaf springs are mounted on opposite sides of the "ice conductor so that the tension forces are the same for both directions of deflection of the conductor :as it vibrates across the surface of the recording field. The deflection of the conductor is directly proportional to the current which flows through it and inversely proportional to the tension on it. Hence, variations in the tension affect the linearity of the galvanometer response to the input current.

Preferably the lower of the pole pieces which provide the magnetic field are provided with recessed or cut out areas arranged so that the length of the conductor upon which the magnetic field operates increases as the central portion of the flexible conductor is deflected in either direction, so as to compensate for variations in the tension in the conductor and provide a substantially linear current-deflection response. This feature of the galvanometer is covered in application Serial Number 461,086, filed on October 8, 1954 by Clifford E. Berry.

The curve of the leaf springs may vary so long as the flexible conductor is properly tensioned by the springs and the springs afford vertical support for the conductor so that the writing pressure of the conductor ,does not vary.

Substantially constant writing pressure may be maintained by use of a bearing sleeve mounted on the intermediate portion of the conductor and used in conjunction with a magnetic writing anvil as disclosed in detail in pending application Serial Number 544,737, filed November 3, 1955 by A. W. Fischer.

In addition to helping to support the flexible conductor in the vertical plane, the flattened portion of the conductor may be used to locate precisely the ends of the conductor. If the extreme ends of the conductor are left unflattened so that a cylindrical portion remains at each tip, the transition from the cylindrical to the flattened portions provides a shoulder which may be registered with respect to the conductor anchor to locate the end of the conductor with respect to the leaf spring.

The conductor anchor is preferably made of electrically conductive material. The flexible conductor is mechanically and electrically connected to the anchor. Flexure of the conductor takes place at the end of the leaf spring remote from the anchor so that the stresses due to flexure are not impressed on the electrical connection. Factors of non-linearity present in conventional string mountings having electrical connections subject to varying stresses are avoided by the suspension means of the invention.

The leaf springs may take either a cantilever form in which the free end of the spring is displaced from its mount and curves near its free end in an arc of approximately or may be a spring of a flat helical configuration of 450 or more. In either embodiment, the outer or free end of the spring is tapered in both of its thickness dimensions so that it is capable of vibrating at a frequency approximately twice that required for the flexi ble conductor and so as to reduce the mass of the springs. The flattened portion of the conductor is cemented to the leaf spring inwardly of the free end of the spring beyond the point of tangency the conductor makes with the curving portion of the. spring at the greatest deflection of the conductor. As the point of tangency changes with varying deflection, the bend part of the conductor shifts and the bending strain is distributed in the conductor, lessening bend fatigue.

The conductor is cemented to the leaf springs to add to the rigidity of the conductor in the plane perpendicular to the deflection of the flexible conductor.

The invention is further explained in the following detailed specification and drawing, in which:

Fig. 1 is a schematic diagram of a direct writing galvanometer;

i ng resistor 30 to the conductive anvil. flows through the current-sensitive recording paper at Fig. 2 is a fragmentary elevation of an embodiment of the invention;

Fig. 3 is a sectional plan view taken on line 3-3 of Fig.4 is an enlargeddetail-plan view of the conductor anchoring means for the embodiment of Fig. 2;

Fig. 5 is an elevational view of the anchoring means of Fig. '4;

Fig. 6 is a sectional plan view similar to the view of Fig. 3 and illustrating a second embodiment of the invention; and

Fig. 7 is a fragmentary elevation of the anchoring means of the embodiment of Fig. 6, taken along line 7-7 ofthat figure.

With reference to Fig. 1, the string element of the galvanom'eter comprises a flexible conductor or string 10 which is supported at its ends by a pair of leaf springs '11 and 12. A base 13supports anchor pins 14, 15' to which springs 11 and 12 are respectively mounted. A conductive bearingsleeve '16 of a suitable ferro-magnetic material is disposed around and swaged or pinched to the center of the string. The springs are arranged to provide resilient support for the string element along the direction of the string, and they serve to restrain movement in all other directions, so that the deflection of the string is not adversely affected by the end supports.

The springs should have a low spring rate to insure constant incremental sensitivity with amplitude, their ef fective mass should be low so that it does not affect the string dynamics in a way which would require an undesirable increase in driving power, and its natural period should be high enough to insure that it follows the string excursions and that there is no tendency for periodic interchange of energy between the string and the springs.

A magnetic pole piece 17 and a pair of members 18 and 20 forming the other pole piece are located on opposite sides of the string element. The upper pole piece is in two parts so as to permit a recording medium to pass between them and adjacent the string element of the galvanometer.

A conductive guide or anvil 22 of a suitable ferromagnetic material is located in the space between the two upper pole pieces 18, 2t) and spaced from each of the two pole pieces. The anvil has an edge 23 which extends perpendicularly with respect to the string element and which is equidistant from the ends of the sleeve. A current-sensitive recording paper 24 moves over the edge ofthe anvil so that it extends between the conductive anvil and the sleeve on the string element of the galvanometer in the plane of motion of the string. The two pole pieces 18, 20 induce magnetic poles of opposite polarity on the anvil, so that a magnetic field is established in the gaps between the anvil and the two pole pieces. The magnetic sleeve is attracted toward each of the gaps between the anvil and pole pieces 18, 20 with a resultant attraction toward the anvil in a direction perpendicular to the plane of deflection of the string.

The ends of the string element are connected to receive a signal from a source 25 through a recording amplifier 26. The electric current which flows through the string element causes its central portion to be deflected in accordance with the magnitude of the signals provided by the signal source. Co-pending application Serial Number 445,518, which was filed on July 26, 1954 by Norton W. Bell, discloses one suitable recording amplifier.

A source of electric current 28 has one terminal con- .nected to the string element of the galvanometer and it has the other terminal connected througha current limit- Thus, current the intersection between the sleeve and the edge of the conductive anvil. Hence, the electric current which flows between the string element and the conductive anvil causes a trace to be recorded on the current-sensitive recording paper in accordance with the movementsof the 4 string element. The bearing sleeve takes any wear which may tend to occur as the string is deflected, and thereby greatly increases the useful life of the string without unduly increasing the effective mass of the string.

Figs. 2 to 5 show the details of one version of the recording apparatus which is capable of recording traces having one inch peak-to-peak amplitude up to a frequency of 250 cycles per second with good linearity.

As before, the string element is a fieexible conductor 46 which is supported by a pair of leaf springs 41 and 42. A conductive bearing sleeve 43 of a suitable ferromagnetic material is coaxially disposed around and soldered to the center of the string. The leaf springs are mounted at opposite ends of a base assembly 44. The base assembly is suspended by means of bolts 45, 46 from a magnet yoke 47.

The magnet yoke supports a pole piece split into right and left pole members 49, 50, respectively. The poles are separated a distance suflicient to allow mounting of a writinganvil 51 which'has a bottom edge 52 contoured to permit the smoothpassage of a current-sensitive recording tape 53 to pass over it between the pole pieces and in direct contact with the conductive bearing sleeve 43. A second lower pole split into separate right and left pole members 54, 55, respectively is situated directly below the pole members 49 and 50. Flexible conductor 40 extends between the upper and lower pole members.

The base assembly has a base strip 56 which extends parallel to the extent of the flexible conductor. The base strip is fastened to an outer side face of magnet yoke 47 by previously mentioned screws 45, 46. Transverse mounting bars 58, 59 extend perpendicular to the base strip in the space between the magnet yoke and the lower right and left pole members 54, 55. One end of each mounting bar is fastened to the base strip by means of a screw 61 and each bar is further secured by means of a key 63 fitted into a groove 64 in the mounting bar and a second parallel groove 65 extending parallel to the mounting bar in the magnet yoke.

Threaded mounting pins 67, 68 of conductive material extend through mounting bars 58, 59, respectively and are secured to the mounting bars by means of nuts 70, 71 threaded onto the bottom portion of the respective mounting pin. Electrical connection is made at these points to the flexible conductor.

The head portion of the-pin is best explained by reference'to Figs. 4- and 5. Each pin has a cylindrical head 73 which has a-slot 74 extending across a chord of the head near its center. A raised square boss 75 is defined in the head by the slot 74 and a narrow groove 76 extending parallelto the slot. A wider groove 78 extends inwardly from the periphery of the head to the edge of the slot and is perpendicular to the slot. The grooves 76 and 78 do not extend as deeply into the cylindrical head as does slot 74 but rather have bottom surfaces which are on a horizontal plane common to a level portion 79 of the head.

The wider groove 78 divides thesquare boss 75 into a longer portion 80 and a shorter portion 81. The shorter portion has a threaded horizontal bore 82 to receive a set screw 83. The leaf spring 42 is illustrated in Fig. 4 mounted in pin 68. An end 85 of the leaf spring is bent at right angles to a stem 86 of the spring. The spring is bent again in a gradual curve to form a free end 87 extending in the same direction as the end 85. Free end 87 is diminished in both width and thickness toward its tip. As mentioned above, this provides a free end with a very small mass and the ability to vibrate in the frequency range required by the flexible conductor. End 85 of the leaf spring resides in groove 76 and is held in place in the groove by a wedge 90. The wedge is kept in the groove by peening the edge of the groove. The stem 86 of the leaf spring extends along the groove 78 and is held in a cantilever position over the flat portion 79 of the head by an'L-shaped clamp 91. One leg of the clamp extends in groove 76 and the other in wider groove 78.

aesmse Flexible conductor 40 is flattened near each of its ends. Flattened portion 93 at the left end of the conductor, as viewed in Fig. 3, extends along the surface of the leaf spring so that the major dimension of the flattened portion is vertical. The flattened portion is interposed and held between the clamp 91 and the leaf spring stem. A cylindrical tip 94 is left on the conductor when its end is flattimed and acts as an index to position the conductor by contacting the clamp 91 and the leaf spring bend. Screw 83 thrusts against a leg of clamp 91 binding the conductor and the leaf spring between the clamp and the face of square boss portion 75.

The flattened portion of the conductor is in electrical contact with the mounting pin, and in mechanical contact with the leaf spring from the clamped spring end 85 to near its free end 87. Since the thickness of the conductor is reduced in the direction of conductor deflection, the conductor bends easily in this direction, while resisting bending in the direction normal to deflection. The vibration of the conductor causes the conductor to bend in that portion nearest the free end of the leaf spring. Since the leaf spring gives due to the added tension caused by deflection of the conductor, the point of tangency of the conductor to the curved free end of the spring changes and shifts the conductor bending point accordingly. Thus, the bending strain is distributed in the conductor and the conductor is less susceptible to fatigue.

Leaf spring 41 is similarly mounted in mounting pin 67 so that its free end is cantilevered away from the pin and curves toward spring 42. flexible conductor has a flattened portion which is arranged with respect to the leaf spring 41 as described for flattened portion 93 with respect to leaf spring 42. Pins 67 and 68 are arranged on the mounting bars 58, 59 so that the leaf springs extend to the pins on opposite sides of the flexible conductor. With respect to the line of the conductor, leaf spring 41 curves away from the conductor toward the lower portion of Fig. 3 and leaf spring 42 curves away from the conductor toward the upper portion of Fig. 3.

When the flexible conductor is not subjected to current flow, it assumes a position midway between the full line position of conductor 40 and the dotted line position 40A in Fig. 3. In this unexcited position the bearing sleeve 43 occupies a central position with respect to the two pole pieces 54 and 55. During the fluctuating current flow through the conductor, the bearing sleeve moves from side to side of a central line 96 as it vibrates from peak to peak of its oscillation. The displacement transverse to the direction of vibration is due to the mounting of the leaf springs on opposite sides of the conductor. This transverse motion tends to distribute the writing wear over the entire surface of the bearing sleeve and is also instrumental in keeping the surface of the bearing sleeve clean.

The lower pole members 54, 55 are cut out adjacent each other as can best be seen in Fig. 3 so that the magnetic field to which the bearing sleeve is exposed increases as the amplitude of the conductor increases. This varying magnetic field helps to compensate for the non-linear effects introduced by the fact that the amplitude induced by current flow varies inversely with the tension on the flexible conductor.

Figs. 6 and 7 illustrate an embodiment of the invention in which the leaf spring has a spiral or helical shape. In Fig. 6 a base assembly 100 supports mounting pins 101, 102. The mounting pins extend through a pair of mounting bars 103, 104 respectively that are supported in a manner similar to that described for Figs. 2 through 5. A helical leaf spring 105 is mounted by one of its ends to mounting pin 101 and a similar spiral leaf spring 106 is mounted by one of its ends to mounting pin 102. The springs curve toward each other. A flexible conductor 108 extends between upper and lower pole members 109, 110 (see Fig. 7) in a manner similar to that described with respect to Figs. 2 through 5. Each end of the flex- The opposite end of the 9 ible conductor is flattened and the flattened portion 111 is wrapped about the leaf spring so that the largest dimension of the flattened portion extends vertically (see Fig. 7). The helical leaf springs are supported by the respective mounting pins so that the free ends of the springs are in contact with opposite sides of the flexible conductor and the leaf springs curve away from the conductor in opposite directions.

A vertical slot in the periphery of each of the pins receives the ends of the leaf springs and the flattened portion of the conductor that is wrapped around the outer curvature of the leaf spring. For instance, the slot 112 in mounting pin 102 has one end of helical leaf spring 106 wedged intoit together with the end of the flattened portion of the conductor.

It has been found that a helical spring which makes approximately 450 of curvature about the mounting pin will give the desired tension to the flexible conductor. As in the embodiment of Figs. 2 through 5,. the ends of both of the helical leaf springs are flattened or diminished in both thickness dimensions to reduce the mass of the spring ends. Each flattened end of the conductor is cemented to its respective spring at a point just back of the point of tangency of the conductor with the spring when the conductor is at its largest amplitude.

In either embodiment the free configuration of the spring is such that the desired tension would be imparted to the flexible conductor when the conductor is fastened to and extended between the oppositely extending springs. For instance, in Fig. 4 cantilever spring 42 has a free configuration indicated by dotted lines 42A. The distortion of the spring under the full vibration amplitude of the conductor is indicated by dotted lines at 42B. The free end of the leaf spring 42 has a curvature of approximately Vs" radius. The flexible conductor is cemented at 114 to this curved portion. The flattened portion of the conductor plus the support gained by being cemented to the leaf spring gives the necessary vertical support to the conductor so that it is restrained from movement in any direction other than the desired plane of vibration.

Since the supporting leaf springs are symmetrically mounted on opposite sides of the conductor, the tensions due to conductor vibrations are equalized for each direction of conductor deflection. The conductor develops considerably less mechanical fatigue due to the unique mounting afforded by our invention. The electrical connections of the conductor are not subject to stresses affecting the linearity of response to electrical input.

The suspension means of the invention is not limited to a single galvanometer conductor string since the support pin mounting bars may be extended through a bank of magnet pole pieces and a series of parallel conductors may be supported by the pins extending through the elongated mounting bars.

We claim:

1. In a string galvanometer in which a flexible conductor is stretched through a magnetic field from anchors on opposite ends of the conductor, and in which variations in voltages impressed on the conductor are manifested by changes in amplitude of movement of the conductor in the magnetic field, the combination which comprises a base, a first and a second leaf spring each having a fastened end and a curving free end remote from the fastened end, the springs being adapted to flex in a plane substantially parallel to the plane of motion of the flexible conductor, the fastened ends of the springs being fastened to spaced points of the base lying on opposite sides of the conductor so that the extent of the first leaf spring from free end to fastened end is opposite to the extent of the second spring from free end to fastened end with respect to the longitudinal path of the conductor, the flexible conductor being fastened at its respective ends to the base adjacent the respective fastened ends of the leaf springs and stretched over the free ends of the leaf springs and extending between them.

2. Apparatus in accordance with claim 1 in which the b'asecompr'ises a base strip parallel to the extent of the flexible conductor, apair of mounting bars, one fixed at either end of the base strip and extending perpendicular to the base strip, and a pair of mounting pins, one anchored in each mounting bar on opposite sides of the extent of the flexible conductor, one of the leaf springs being secured to each mounting pin.

3. Apparatus in accordance with claim 1 in which the base comprises a base strip paralIelto the extent-of the flexible conductor, a'pair of mounting bars, one fixed at either end of the base strip and extending perpendicular to the base strip, a pair of electrically conductive mounting pins, one anchored in each mounting bar on opposite sides of the extent of the flexible'con'ductor, each pin being grooved toreceive'a'n end of "the conductor and a fastened end of a leaf spring.

4. Apparatus in accordance with claim 1 in which each of the leaf springs is a narrow blade disposed in part in a helix about its fastened end, the free ends "of the respective helixe's being curved toward each other.

3 '5. Apparatus in accordance with claim -1 in which the flexible conductor between the free ends of the leaf springs and the fastened ends of the leaf springs is of flattened cross'section, the flattened sides of the conductor lying against the sides of the leaf springs so that the bending fatigue is lessened by the shifting of the bend ing point of the flexible conductor at each leaf spring as the conductor is deflected. 

